The present invention is a method of measuring edge profile of one or more edges of an elongated member, such as a piece of lumber, as the member advances past a scanning station on a conveyor. The method is particularly well adapted for use when transport conditions induce translational and/or rotational movement in the member being measured relative to an arbitrary baseline located generally parallel to the longitudinal axis or centerline of the member. The method further includes estimating deviation from linearity over any given length of the member for quality control purposes.
The knowledge of any deviations from linearity is an important aspect of quality determination in elongated members such as metal structural elements, lumber or sawn timbers. Lumber may be taken as exemplary. Virtually every one of the untold millions of pieces of lumber entering the market place are individually examined by a skilled quality control inspector. Each piece is assigned to a grade determined by rules which are recognized on an industry-wide basis. Graders working in sawmills are subject to certification and inspection by head graders from industry-sponsored associations. Their job involves high skill and requires the ability to internalize all of the complex grading rules and assign a proper grade to lumber after a visual inspection made within a few seconds time.
Lumber is graded using rules dependent on its intended application. Boards intended for remanufacture into items such as millwork or furniture are graded primarily on visual appearance and defects which might affect structural strength are given only minor consideration. On the other hand, appearance is of much less importance in lumber intended for structural uses and defects which affect strength are given heavy weighting. The size of knots, expressed as their estimated cross sectional area, and their location in a piece of lumber are of primary consideration. Other defects such as shake, splits, decay, or density are also considered. Defects which may be either of visual or structural importance, such as planer skip and wane, also enter into grading rules.
Warpage is another type of defect which, while it does not affect structural strength, is considered in grading. Warpage is generally a combination of one or more of four types: twist, bow, cup or crook. Twist is caused by cross sectional rotation along the length of the piece. Crook is deviation from linearity of the edges of a piece of lumber when it is laid on one of its widest faces. Cup is lack of planarity across the faces at a given point. Bow is warpage 90.degree. displaced from that identified as crook and is a deviation from end-to-end planarity of the faces. When laid on one of its faces, bow causes the lumber to be shaped like the rocker of a rocking chair.
The grading rules set allowable limits within any given grade for all of the above defects. Currently these are judged and measured only by the skilled eye of the grader. All can be precisely measured using conventional measuring tools with reference surfaces such as flats or straight edges. However, this it not possible in the few seconds available to a grader and visual estimates must suffice.
As one example, the Southern Pine Inspection Bureau, an industry sponsored grading association, has established the following limits for crook in southern pine dimension lumber.
______________________________________ Permissible Crook 4 ft (1.22 m) 20 ft (6.10 m) ______________________________________ No. 1 3/16 in (4.8 mm) 1 1/32 in (28.2 mm) No. 2 1/4 in (6.4 mm) 13/8 in (34.9 mm) No. 3 3/8 in (9.5 mm) 2 1/16 in(52.4 mm) ______________________________________
It should be noted that the limits listed under 4 ft (1.22 m) do not apply to pieces of lumber only four feet long, but to any possible four foot section within a longer piece. The same is true of the values listed in the 20 ft (6.10 m) column. Very obviously a heavy, if not impossible, burden is placed on the grader to estimate within such precise tolerances. Even so, graders are expected to maintain a minimum of 95% accuracy in assignment of lumber to proper grade.
The technology to enable machine grading or machine assisted grading has been relatively slow to develop, especially for structural grades of lumber. Many of the automated grading systems proposed to date use gray scale-type scanners which attempt to recognize knots by their darker colors. Most of these systems also require some form of human input. Idelsohn et al, U.S. Pat. No. 4,207,472, can be considered as exemplary of electro-optical grading. Dahlstrom, in U.S. Pat. No. 3,983,403, teaches a method for recognition and measurement of wane on lumber. Wane is a condition where a corner is not square but is truncated by a portion of the original log surface. Matthews et al, in U.S. Pat. No. 4,606,645, disclose a scanner which can measure grain direction relative to three mutually orthogonal axes. This scanner marks a major advance over those previously known for grading lumber based on characteristics which affect its structural strength.
In addition to being visually graded, some lumber is also machine stress rated. This involves bending the piece, usually as a plank, to determine modulus of elasticity in flexure. If the estimated M.O.E. is below some minimum value, the lumber is placed in a lower grade. Porter et al, in U.S. Pat. No. 4,589,288, disclose such a method particularly adapted for use with plywood.
A number of methods have been proposed for measuring sweep in logs to assist in making bucking and sawing decisions. Sweep is end-to-end curvature and may be considered as analogous to crook in lumber. Typical of these are U.S. Pat. No. 4,548,247 to Eklund and U.S. Pat. No. 4,640,160 to Hards. In Eklund a curved or sweepy log is first mechanically sensed and processed to yield a curved cant by creating faces parallel to the centerline of the log. The cant is then resawn along a similar curved path parallel to the faces. Hards discloses a scanning system which examines log length, diameter, and sweep in order to make bucking decisions prior to sawing. However, this inventor describes the scanners only as "one of a large variety of commercially available laser-scanning devices" and offers no more information. Both of these systems operate at relatively low speed. Hards discloses operating speeds in the range of 45-75 m/min. Pirlet et al, Iron and Steel International, August 1978, pp. 215-221, describe a method for measurement of flatness of a steel strip. This is done by measuring the center and edge heights at given time interval using a laser and photodiode camera arrangement. A computer then calculates flatness indices which can be used for process control. The system compensates for any vertical motion on the strip by presuming the motion to be the same at the center and both edges.
None of the systems noted would be useful for measuring crook in lumber or similar elongated members traveling at the high speeds normally found in modern sawmills. The present inventors believe that theirs is the first method capable of accurately measuring edge profiles in elongated structural members, such as lumber, at speeds which may be typically in the range of 300-350 m/min.